Disclosed in JP2005-297960A is a seat cushion frame for a vehicle having a left and right cushion side frames (i.e. first and second back frames). JP2005-297960A states that the left and right cushion side frames are adaptable as left and right back-side frames, respectively. Accordingly, the seat cushion frame for the vehicle according to JP2005-297960 will be explained below with the left and right back-side frames. Each of the left and right back-side frames has a shape, which is obtained by bending both end portions of a plate member in order to form two boxes. Furthermore, a recessed portion is formed between two boxes of each of the left and right back-side frames. The both end portions of the plate member of each of the left and right back-side frames are connected to the main body thereof at which the recessed portion is formed in a state where the end portions of the plate member face each other in order to enhance a strength of the seat back frame for the vehicle.
Disclosed in JP2005-67331A is a seat back frame for a vehicle having right and left back-side frames (first and second back frames), an upper cross member for connecting the right and left back-side frames at upper portions thereof and a lower cross member for connecting the right and left back-side frames at lower portions thereof. Each of the right and left back-side frames is formed in a box shape. Each of the upper cross member and the lower cross member is formed in a tubular shape. The lower cross member is connected to the right and left back-side frames by welding in a state where both end portions of the lower cross member are inserted into holes formed at right and left back-side frames, respectively, in order to ensure a strength at the seat back frame for the vehicle.
Disclosed in DE10048128A1 is a seat back frame for a vehicle having right and left back-side frames, an upper cross member for connecting the right and left side frames at upper portions thereof and a lower cross member for connecting the right and left side frames at lower portions thereof. Each of the right and left back-side frames is obtained by bending both end portions of a plate member into substantially cylinder shapes. Furthermore, the upper cross member is formed in a tubular shape. The lower cross member is formed in a bar shape. The right and left back-side frames are arranged so that plate surfaces thereof face each other. Furthermore, both end portions of each of plural lower cross members are connected to circumferential surfaces of cylindrical-shaped portions of the right and left back-side frames positioned at a back-side in front-rear direction of the vehicle, respectively, in order to ensure a strength at the seat back frame for the vehicle.
Disclosed in DE102004048351A1 is a seat back frame for a vehicle formed as a unit. A portion of the seat back frame for the vehicle disclosed in DE10200404835A1 corresponding to the lower cross member is formed in a substantially flat surface shape in order to ensure a strength at the seat back frame for the vehicle.
Generally, in a case where a shoulder belt anchor for supporting one end portion of a seat belt is provided at an upper portion of the seat back frame for the vehicle, if the vehicle is involved in a crash, the seat belt is pulled forward by a great tensile force because of an inertia generated by an occupant, thereby applying a great load to the shoulder belt anchor. For example, the seat belt is arranged so as cross the passenger's body from the right shoulder to the left waist and then to the right waist. Therefore, in the case where the vehicle is involved in the crash, a diagonally forward left-torsional bending moment is generated at the seat back frame for the vehicle because of the above-mentioned load. A similar situation may occur at a seat for two people used as a back seat for the vehicle.
In a case where the configuration of the back-side frame of the seat back frame for the vehicle disclosed in JP2005-297860A is adapted as a center back-side frame, a compression deformation may occur at the center back-side frame with the recessed portion as a fiducial point when the torsional bending moment acts on the center back-side frame, because two box shaped portions are divided at the recessed portion, to which the end portions thereof are connected. On the other hand, in a case where the configuration of the back-side frame of the seat back frame for the vehicle disclosed in JP2005-67331A is adapted as the center back-side frame, a compression force acts on a front wall portion of the plate shaped member, which is bent to have a box shape and which is arranged at a front portion of the seat in the front-rear direction of the vehicle, because of the torsional bending moment. Therefore, a compression deformation may occur at a connected portion at the front wall portion because of the compression force acting on the front wall portion, at which end portions of both side end portions of the plate-shaped member bent to form a box is overlapped and connected.
According to the seat back frames disclosed in JP2005-297960A and JP2005-67331A, the center back-side frame may be formed to have a curved shape, so that a recessed portion is formed at a rear portion of the seat in order to provide a comportability for the occupants seated on the seat for two, furthermore, in order to simplify the configuration of the center back-frame.
In the case where the plate-shaped member of the center back-side frame having the configuration of the back-side frame disclosed in JP2005-297960A is formed to have a curved shape so that the recessed portion is formed at the rear portion of the seat, a tension acts on the side end portions of the plate-shaped member, which form the box-shaped portion at the rear portion of the seat. Therefore, flatness of the plate-shaped member may be achieved with accuracy, which may result in facilitating a connection between the recessed portion and the both end portions of the plate-shaped member. However, because the side end portions of the plate-shaped member forming the box-shaped portion are compressed when being processed, unevenness or crinkle is likely to generate on surfaces of the side end portions of the plate-shaped portion. Accordingly, the flatness of the side end portions of the plate-shaped member may not be obtained with high accuracy, which may result in causing a difficulty in connecting the end portions of the plate-shaped member with the recessed portion.
The lower cross member of the seat back frame for the vehicle according to JP2005-67331A may have a rigidity against the torsional bending moment because the lower cross member is formed in the tubular shape. However, the lower cross member and the back-side frame are connected in a manner where an outer circumferential surface of the tube-shaped member contacts an inner circumferential surface of a hole formed at the plate-shaped member, in other words, the lower cross member and the back-side frame are connected along a line (i.e. a connected line). Therefore, in a case where a tension is concentrated on the connected portion between the lower cross member and the back-side frame because of the torsional bending moment, a compression deformation may occur at the connected portion.
The lower cross member of the seat back frame for the vehicle according to DE10048128A1 is formed in the bar shape. Therefore, a rigidity of the lower cross member against the torsional bending member may be lower than the lower cross member formed in the tubular shape. However, because plural lower cross members are provided at the seat back frame for the vehicle disclosed in DE10048128A1, a load applied thereto may be dispersed. The back-side frame and each of the lower cross members are connected in a manner where an outer circumferential surface of the bar-shaped portion contacts an outer circumferential surface of the cylindrical member, in other words, the back-side frame and each of the lower cross members are connected along a line (i.e. a connected line). Therefore, in a case where a tension is concentrated on the connected portions between the lower cross members and the back-side frame because of the torsional bending moment, the connected portions may fracture. Furthermore, according to DE10200404835A1, because the seat back frame is formed as the unit, a load applied thereto may be dispersed. However, the portion corresponding to the lower cross member is formed in a substantially flat surface shape. Therefore, if the seat back frame for the vehicle is even slightly deformed, the seat back frame for the vehicle may be suddenly compressingly deformed.
A need thus exists to provide a seat back frame for a vehicle which is not susceptible to the drawback mentioned above.